We provide a service to other investigators who need pregnant monkeys of known gestational ages and to monitor fetal growth in rhesus monkeys (Macaca mulatta). The monkeys' cycles are artificially stimulated to induce a luteal phase by the daily injection of progesterone (5 mg/day) for 10 days. Females are paired with males on days 14 to 21 after the last progesterone injection. Past studies indicate that ovulation and conception occurs on day 16 to 18 after the progesterone injections. Success rate, based on viable fetus at 21 days gestation, averages from 60% to 85% for first cycle matings, depending on age and health status of the females. Fetal development is monitored by weekly ultrasound exams until time of parturition. At each exam measurements are taken, depending on gestational age, to determine gestational sac length, crown-rump length, heart rate, femur length, and bipartial diameter. These measurements allow for determination of fetal age and of deviations from normal growth parameters. Evaluation of the placental disks are made to determine any anomalies. These procedures provide investigators with the ability to conduct studies at specific stages